1. Field of the Invention
The present invention relates to a proton conductive polymer, a catalyst composite, an electrolyte membrane for a fuel cell, and a fuel cell.
2. Related Art
It is in public that the proton conductive polymer is useful as a functional polymer, since it could be used as an ion exchange resin or a solid acid catalyst. Recently, researches are being commonly made that an electrolyte membrane comprising the proton conductive polymer could be applied to a fuel cell.
The fuel cell is a system for directly transforming chemical energy of fuel into electrical energy by electrochemically oxidizing a fuel e.g. hydrogen or methanol in the fuel cell and outputting the latter energy. The system is now drawing an attention in view of a clean electrical energy source, because the system outputs no NOx nor SOx, which are output by conventional power plants combusting fossil fuels. Especially, the proton conductive polymer fuel cell can operate with very high efficiency, because the fuel cell is free from thermodynamic restrictions of Carnot cycle. The theoretical efficiency of the proton conductive polymer fuel cell reaches as much as 83% at 25 degree.
Polymer bearing a sulfonic acid group is known as the proton conductive polymer being used with the proton conductive polymer fuel cell. One of representative examples of the sulfonic acid group containing polymer is “Nafion” (registered trademark) developed by and commercially available from Du Pont Inc or equivalents thereof “Nafion” (registered trademark) includes a copolymer of tetrafluoroethylene and perfluorovinylether as a base and the sulfonic acid group as an ion exchange group. The polymer is a chemically stable compound.
For manufacturing the fuel cell, an electrode complex (membrane electrode assembly (MEA)) is formed by processes of that “Nafion” (registered trademark) is held between an anode and a cathode and further the assembly is hot-compression bonded by hot pressing. When it comes to power generation of the fuel cell, fuel are supplied to an anode end of MEA−, while oxidizing agents (e.g. air or O2) are supplied to the cathode end thereof. The proton and the electron are electrochemically produced at the anode end. The electron produced flows through an external circuit. On the other hand, the proton reaches the cathode end through “Nafion”(registered trademark), where the proton and the electron terminating from the external circuit react with each other via the oxidizing agent and leads to water production. As results, the electrical power is generated.
A certain problem will, however, occur, in case of that “Nafion” (registered trademark) or the equivalents thereof is used with the direct methanol fuel cell. Specifically, the methanol having permeated through the anode disperses in “Nafion” (registered trademark), and reaches the cathode, where directly reacts with the oxidizing agent on the cathode catalyst, this phenomenon is referred to “cross-over”, resulting in very low fuel cell performance.
Laid-open patent Hei9-73908 and Laid-open patent 2000-195528 disclose a composite membrane which includes strong acid immersed in polybenzimidazole as the proton conductive polymer other than “Nafion”(registered trademark). In addition to that, Polymer Preprints, Vol. 51, No. 4, P 749 IPg 141 discloses a composite membrane including poly(1,2,4-triazole) and phosphoric acid. The entire contents of which are incorporated herein by reference.
These composite membranes mentioned above have imidazole or (1,2,4-triazole) in its main chain skeleton, which retains the strong acid. This leads to exhibiting proton conductivity. The polymer contained in the membranes has, however, rigid skeleton. The proton of the strong acid retained in the skeleton is believed to be poorly conductive, because of its low mobility.